Intersection & Interchange Geometrics (IIG)
Innovative Design Considerations for All Users
Module 1
Introduction

Mark Doctor
Atlanta, GA
(404) 562-3732 mark.doctor@dot.gov
George Merritt
Atlanta, GA
(404) 562-3911 george.merritt@dot.gov
FHWA Office of Technical Services
Resource Center Safety & Design Team
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 Your Name, Organization & Position
 What you would like to learn from this workshop
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 Please silence cell phones
 Please ask questions
 Breaks / Lunch
 Sign-in sheet &
Registration Form
 Post-test
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Module 1 - Introduction
 Define what is meant by “innovative” or
“alternative” intersections and interchanges
 Describe their potential advantages
 Define “intersection conflict points” and apply the concept to assess potential safety differences among intersections
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WHY DO WE NEED INNOVATIVE
INTERSECTION &
INTERCHANGE DESIGNS?
Module 1
Introduction

WHY ALTERNATIVE INTERSECTIONS &
INTERCHANGES?
Visualize a high volume intersection or interchange you have travelled through recently
What do you see? Who do you see?
???
Insert mental image of your intersection here
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WHAT ARE THE PROBLEMS?
• Increasing Congestion
• Too Many Crashes
• Quantity and severity
• Safety for ALL users
• Not Enough Funding
• Time Consuming Projects
• Inability for more right-of-way
• Minimize the impacts of projects
• Environmental, social, economic
Source: Mark Doctor, FHWA
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WHY INNOVATIVE INTERSECTIONS &
INTERCHANGES?
Intersections are usually the bottlenecks along high volume roadways and are often a safety concern
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Photo Source: Mark Doctor, FHWA

INTERSECTION SAFETY FACTS
Angle crashes account for over 40% of fatal crashes at intersections
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INTERSECTION SAFETY FACTS
Left turn crashes account for over 20% of fatal crashes at intersections
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INTERSECTION SAFETY FACTS
Ped/Bike crashes account for 25% of fatal crashes at signalized intersections
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INTERSECTION CONFLICT POINTS - VEHICLES
16 Crossing
Conflicts

INTERSECTION CONFLICT POINTS –
VEHICLES/PEDS
Four vehicle/pedestrian conflicts per crossing
Conflicts involving
RTOR and
“permissive” left turns can be particularly problematic
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WHAT ARE ALTERNATIVE
INTERSECTIONS &
INTERCHANGES?
Module 1
Introduction

WHAT ARE ALTERNATIVE
INTERSECTIONS & INTERCHANGES?
Innovative designs that:
• Improve the way traffic makes certain movements by eliminating, relocating or modifying conflict points
• Strategically improve signalization
Source: Mark Doctor, FHWA
Source FHWA
Source: Missouri Department of Transportation
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SIGNAL PHASING
Basic twophase signal operation
Source: MnDOT Traffic Signal Timing and Coordination Manual

SIGNAL PHASING
Adding “protected” left-turn phases is common as volumes increase
Source: MnDOT Traffic Signal Timing and Coordination Manual

SIGNAL PHASING
Adding more phases essentially “steals” time away from the major through movement and can increase intersection delays

SIGNAL PHASING
Minor
Street
Minor Street Major Street Major Street Major Street
Strategically relocating left turn movements can provide more green time to through traffic

IN SUMMARY …
Fewer phases – GOOD / Left turns - BAD
Photo: 2-Phase operation at a Median
U-turn Intersection

BENEFITS OF INTERSECTION &
INTERCHANGE GEOMETRICS
SAFETY
• Fewer conflict points
• Significant
Before/After
Crash
Reductions
MOBILITY
• Less delay
• Reduced congestion
VALUE
• Less ROW
• Less construction costs
• Implemented quicker

Module 1 - Introduction
 Define what is meant by “innovative” or
“alternative” intersections and interchanges
 Describe their potential advantages
 Define “intersection conflict points” and apply the concept to assess potential safety differences among intersections
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Module 1 - Introduction
 What are “innovative” or “alternative” intersections and interchanges?
They are designs that:
• Improve the way traffic makes certain movements to eliminate, relocate or modify conflict points
• Strategically improve signalization
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Module 1 - Introduction
 What are the potential advantages of innovative intersections & interchanges?
• Safety
• Mobility
• Value
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Module 1 - Introduction
 Define “intersection conflict points” and apply the concept to assess potential safety differences among intersections
At a conventional four-leg intersection, the combination of various through and turning movements create a total of 32 points of conflict. These conflict points can be classified into three different types: crossing, merging, and diverging.
Crossing maneuvers occur where vehicle travel paths cross one another, merging maneuvers occur where vehicles from one traffic stream enter into another, and diverging are located at points in which vehicles depart a traffic stream.
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